In many presently available electrical or electronic measuring systems it is necessary to manually calibrate the system for voltage offsets in the transducers and in the electronics. Furthermore, it is necessary to manually calibrate the systems for gain due to environmental conditions such as temperature which have an effect on the circuit elements of the measuring system. Typically, this is the time-consuming job and the accuracy of the calibration process depends upon the operator. These problems are further magnified when many measuring systems are to be calibrated at the same time. One calibration method includes balancing a high impedence difference amplifier wherein the operator observes a meter and adjusts the bounds until the meter nulls. Other inaccuracies introduced into the measuring system include drift and changes in reference voltage.
U.S. patents to Berry, U.S. Pat. No. 2,734,949, Schramm, U.S. Pat. No. 2,767,375, Cole et al, U.S. Pat. No. 2,919,401 and Senour U.S. Pat. No. 3,667,041 all disclose automatic calibration systems or circuits. The automatic calibration systems calibrate unipolar measuring devices, that is, measuring devices which measure and output a voltage between zero volts and a predetermined number of volts.
U.S. patent to Petersen et al U.S. Pat. No. 3,934,197 discloses an automatic calibration system for ph meters including a digital counter and a digital to analog converter which produces an output voltage connected to a difference amplifier to compensate the zero offset D.C. voltage of the ph measurement system. The automatic calibration system is applicable to unipolar measuring devices.
U.S. patent to Ries et al, U.S. Pat. No. 3,454,880 discloses a transformation system for periodic compensation of inaccuracy arising from sensor instability. A measuring device measures preselected reference quantities which are compared with preset electrical reference signals whereby difference voltages derived from the comparison are used to control adjusting means which adjust the transformation characteristics of the system including the zero point and the reference voltage which corresponds to the end of the measuring range.
Other calibrating circuits generally of the type to which this invention relates are disclosed by the U.S. patents of Sprong U.S. Pat. No. 1,090,419, Kuckein, U.S. Pat. No. 3,633,202, Allington et al U.S. Pat. No. 3,667,056, MacKnight et al U.S. Pat. No. 2,787,418, Bell U.S. Pat. No. 2,874,354, Alexander et al U.S. Pat. No. 2,897,486, Ullrich U.S. Pat. No. 3,181,063, Goransson, U.S. Pat. No. 3,535,637 and Petrohilos U.S. Pat. No. 3,665,305.